Proximal record of the 273 ka Poris caldera-forming eruption, Las Cañadas, Tenerife

Abstract

Lithofacies architecture analysis is applied to the most proximal exposures of the 273 ka Poris Formation, in the Diego Hernandez caldera wall of Las Canadas. Here, the succession records an opening Plinian phase followed by generation of a pyroclastic density current (PDC) that was partly blocked by near-vent topography. The upper part of the current surmounted the barrier and deposited an ignimbrite veneer. During a hiatus in energetic flow, ash and various ash aggregates were deposited from a hybrid cloud comprising parts of both the Plinian and the co-ignimbrite plumes. Phreatomagmatic explosions then generated currents rich in fine-grained lithics; current waxing with local erosion was followed by pulsing-unsteady waning, recorded in repeated sets of graded tuff. Reversion to magmatic activity was marked by hybrid deposition, as coarse pumice fallout and juvenile ballistics entered pyroclastic currents to form stratified pumice-block tuff. Increasingly energetic flow is recorded in 9 m of mainly massive lapilli tuff that includes evidence of widespread scour and bypass; lithic-rich layers record incremental onset of edifice disruption due to magma withdrawal. Laterally variable stratification and load structures show that the PDC was unsteady and highly non-uniform at times, with locally rapid aggradation of gas-rich ignimbrite. Climactic edifice disruption is marked by a lithic-block layer in erosional contact with the underlying ignimbrite, recording intense pyroclastic current activity and widespread bypass of material to distal volcano slopes. Pumice-block breccia, 10 m thick and with blocks up to 1.2 m in diameter, forms the top of the succession and registers modification of the magmatic plumbing. The timing of collapse to form a major summit depression is uncertain. However, the abrupt termination of the proximal aggradation of coarse breccia, with no record there of any waning-stage pyroclastic currents, suggests subsidence at that time, conceivably with formation of a caldera that contained late-erupted material.